How Astrolabes Work

Long before clocks and calendars, humans turned to the heavens to measure time and orient themselves on the planet. Ancient people observed cycles related to the motions of Earth, the sun and the moon and used these cycles to determine the length of days, months and years. They also watched the stars with great interest, arranging them into pictures -- constellations -- as a way to bring order to the mad chaos of the night sky.

Another organizing convention was the "celestial sphere," an imaginary globe thought to surround Earth. Like a traditional globe, the celestial sphere possessed north and south poles, an equator and coordinates similar to latitude and longitude. To an observer on Earth's surface, stars existed as fixed points of light on the inside of the sphere. The sun, moon and planets weren't fixed to the sphere, but moved around on a circular path known as the ecliptic.

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Now imagine that you wanted to take the three-dimensional celestial sphere and project it onto a flat, two-dimensional surface. This was the fundamental problem that confronted scholars like Hipparchus, who was born in Nicaea in 180 B.C. Hipparchus kept meticulous records of 850 stars, an activity that led to the discovery of precession (wobbling of Earth on its axis) and to a unique way of describing a star's position. The Greek astronomer was able to construct a map by imagining a perpendicular line connecting each star to a point on a plane corresponding to the plane of the Earth's equator. The map, which preserved the angular relationships among the stars, may have been the first example of stereographic projection.

Claudius Ptolemy drew heavily from Hipparchus as he prepared his magnum opus, the "Almagest," and other books. In "Planisphaerium," published in 150 A.D., Ptolemy provides a complete description, almost certainly based on ideas from Hipparchus, of the mathematical techniques required to project points on the celestial sphere. The book seemed to be a handbook to construct a working instrument, but no evidence exists suggesting he actually built an astrolabe. He did, however, design and build the armillary sphere, a complex predecessor of the astrolabe

The first authoritative account of what would become the modern, much-easier-to-use astrolabe came from Theon of Alexandria in 390 A.D. OK, so Theon didn't actually build an astrolabe, but historians think he did provide a full blueprint.

When Islamic astronomers picked up Theon's treatise on astrolabes, they saw their value immediately. They began making and using the instruments -- and composing their own manuals. The first astrolabe guides written in Arabic appeared in the eighth century. By the 11th century, the devices began appearing in Muslim Spain. From there, it was hop, skip and a jump into Christian Europe, where astrolabes helped astronomers -- and even gifted poets like Chaucer -- bring order and stability to the night sky. They were an indispensable tool throughout the Middle Ages, until they became supplanted by newer, more specialized technologies, such as telescopes, sextants and pendulum clocks.

Planispheric or Maritime, Madam?

Zip back to the ancient world and you'd encounter two basic types of astrolabes. The first type, known as a planispheric astrolabe, helped astronomers calculate the positions of celestial objects. All early astrolabes were of the planispheric sort until seafaring folks saw how useful the instruments could be.

By about the 15th century, maritime astrolabes began appearing. They were like hacked versions of their planispheric cousins, used primarily to determine the altitude of the sun or a star, which could then be used to determine latitude. They came with two basic parts -- a graduated circle and an alidade, a sighting device or pointer used to measure angles. Planispheric astrolabes were a bit more complicated. They were also more idiosyncratic because their operation depended on the user's latitude.